1. Field of the Invention
The invention relates to surgical apparatus. More particularly, the invention relates to an apparatus for determining the degree of femoral rotation in flexion and for accurately locating a cutting block for resection of the medial and lateral posterior femoral condyles as well as a method for using the apparatus.
2. Description of the Related Art
Total knee arthroplasty involves the replacement of portions of the patellar, femur and tibia with artificial components. In particular, a proximal portion of the tibia and a distal portion of the femur are cut away (resected) and replaced with artificial components. In order to properly place the artificial components, the femur and tibia must be resected in a relatively precise manner so that the artificial knee is balanced throughout its range of motion. Modern knee prostheses are now designed to ensure that so long as the prosthetic knee is balanced in the flexion (90 degrees) and extension (0 degrees) positions, it will function properly throughout its full range of motion.
It is normal for a natural knee to exhibit irregular alignment due to soft tissue differences in the medial and lateral compartments. Differences in the soft tissue, e.g. collateral ligaments, will allow a certain degree of outward (valgus) rotation of the femur relative to the tibia when the knee is in flexion and will allow a certain degree of inward (varus) bending of the tibia and valgus bending of the femur when the knee is in extension. If the artificial knee is not balanced to compensate for these variations in collateral ligaments, it will wear excessively on one condyle, the components may loosen, and most importantly, the patient will experience discomfort.
The traditional methods used to balance the knee relied primarily on x-rays and the surgeon's own senses. Imbalance in the knee when the knee is in extension may be accurately ascertained by x-ray examination. However, imbalance during flexion cannot be ascertained by x-ray examination because the collateral ligaments are contracted during flexion. Traditionally, imbalance during flexion was estimated by visual inspection and palpation of the ligaments during surgery. More recently, various tools referencing bony landmarks have been made available to aid the surgeon in determining the amount of imbalance when the knee is in flexion.
According to an exemplary state of the art procedure, the natural knee is first examined in extension. The potential varus/valgus conditions are assessed with the aid of an x-ray prior to performing the distal femoral and proximal tibial resection. Cutting blocks are attached to the femur and the tibia with drill pins and the proximal tibia and distal femur are resected using either classical or anatomic alignment methods.
The difference in the two alignment approaches is due to a difference in opinion among specialists. The objective of classical alignment is to create a prosthetic joint line which is perpendicular to the reconstructed mechanical axis of the joint. Classical alignment specifies a neutral tibial cut of 0 degrees varus and a valgus femoral cut of 5 degrees-7 degrees. The objective of anatomic alignment is to reproduce a joint line which is parallel to the ground with a normal gait pattern. Anatomic alignment specifies a tibial cut which is made in a plane having a varus angle of 2 degrees-3 degrees and a valgus femoral cut of 7 degrees-9 degrees. Resecting at these respective angles assures that the resected distal femoral condyles lie in a plane which is parallel to the resected proximal tibial surface when the knee is in extension.
The procedures used to balance the knee in the flexion position are more difficult and less accurate than the procedures for balancing in the extension position. In order to balance the knee in flexion, the posterior femoral condyles must be resected so that they lie in a plane which is parallel to the resected proximal tibial surface when the knee is in flexion and the surrounding soft tissues are in balance. Some presently used procedures include referencing the angle of the posterior condylar resection to the classical or anatomic alignment modes used in extension balancing, resecting the condyles parallel to the epicondylar axis, or parallel to the posterior condylar axis.
The angle of resection is measured relative to the center of the two condyles using a jig which has two posterior skids which are placed under the posterior condyles and which orients the jig at 0 degrees rotation relative to the coronal plane of the femur. Drill holes in the jig (or drill bushings attached to the jig) reference an angle of rotation of the femur. The drill holes are used to drill into the resected distal end of the femur so that a cutting block may be attached at a selected angle for resecting the posterior condyles.
None of these procedures accounts for the presence of soft tissues and ligamentous structures. The presence of these tissues and structures tends to negate the assumed correlation between extension balancing and flexion balancing. In particular, the posterior capsule and surrounding soft tissues, which are taut when the knee is in extension, collapse or become laxed when the knee is in flexion and therefore have more significant impact on flexion balance. While the amount of error in flexion balancing due to the presence of these tissues and structures may only amount to a few degrees, it is enough to result in a significant imbalance in the prosthetic knee.
In order to accurately assess the impact of soft tissues on the balance of the knee in flexion, it is necessary to tension the tissues. Several tensioning devices are known in the art which allow the surgeon to spread the femur and tibia apart so that the collateral ligaments may be inspected when the knee is in flexion. Generally, it is found that the medial ligaments are either shorter or have greater tensile strength than the lateral ligaments which results in a valgus rotation of the femur. Most of the known tensioning devices do not provide any means for measuring the degree of rotation and this determination is made with the surgeon's own senses. If it appears that there is a valgus rotation of more than 3 degrees, the surgeon may elect to partially release the medial ligaments which tends to lessen the amount of rotation. If it appears that the amount of rotation is less equal to or less than three degrees, the ligaments are generally left untouched. If it appears that the amount of rotation, according to the indicia, is in a negative range (less than 0 degrees) the surgeon may elect to release the lateral soft tissues until the femur is in an acceptable range of internal rotation (0 to 3 degrees) In any case, the posterior condyles are then resected using a drill jig and cutting block as described above.
U.S. Pat. No. 5,468,244 to Attfield et al. discloses a tensioning device which includes a means for measuring the amount of femoral rotation when the knee is in flexion with the ligaments tensioned. Generally, the device has a tibial engaging plate and a pivoting femoral engaging plate. The plates are arranged between the tibial plateau and the posterior condyles of the femur. The plates are displaced relative to each other and the femoral engaging plate rotates about a central axis in response to femoral rotation. A scale is provided to indicate the angle of rotation of the femoral engaging plate.
The Attfield et al. device is useful in assessing the amount of femoral rotation, but it is not entirely accurate. In particular, the instrument is imprecise because it measures femoral rotation about an axis which lies between the medial and lateral condyles. In reality, the internal (valgus) rotation of the femur is not about a centrally located axis, but is about an axis closer to the medial condylar compartment. In addition, the device provides no assistance in mounting or choosing a cutting block for posterior condyle resection.